1. Field of the Invention
The present invention relates to bearings. More particularly, the present invention relates to a preload hydrostatic bearing with a diaphragm for improved performance.
2. Description of the Related Art
X-Y stage systems are typically used in machine tools and other applications where two-dimensional precision movement is required to position an object supported on the stage. A typical X-Y stage system includes a pair of parallel-spaced guide rails and a stage with at least one fixed air bearing at one end and a corresponding preload air bearing at the other end. The fixed and preload air bearings ride along respective guide rails to move the stage therealong. The preload air bearing provides a constant force to the fixed air bearing and maintains a constant air gap or flying height in the fixed air bearing.
Because it is difficult for guide rails of stage systems to be perfectly uniform, a preload air bearing must compensate for variations in the guide rails, due to thermal growth or other causes, while providing a constant force to the fixed air bearing. Conventional air bearings utilize mechanical preload devices including combinations of ball bearings, conical bearing seats and spring washers, such as Belleville washers, to compensate for rail variations. Examples of these air bearings may be found in U.S. Pat. No. 4,191,385, issued Mar. 3, 1980 to Fox et al. and U.S. Pat. No. 4,882,847, issued Nov. 28, 1989 to Hemmelgarn et al. FIG. 1 illustrates one such prior art preload air bearing 100. Bearing 100 includes a pad 102 having a bearing surface 103. Pad 102 is coupled to a cap 104. Bearing pad 102 is made of a porous material, such as graphite. In the alternative, pad 102 may have a plurality of orifices formed therein. Cap 104 has an internal space for receiving a compressed gas, such as air, from an external source. The compressed gas flows through cap 104 and pad 102 to create an air film between bearing surface 103 and a rail surface (not shown) on which bearing 100 rides. A ball 106 which is received in a seat 144 supports bearing cap 104. A spring washer 148, or stack of spring washers, supports seat 144 and ball 106. Washer 148 is secured on a boss 150 at one end of a preload pin 146. The arrangement of ball 106, seat 144 and spring washer 148 allows bearing cap 104 and pad 102 to tilt and accommodate slight variations in the rail surface. The air film gap may be altered by adjusting the position of preload pin 146.
One problem with conventional air bearings, however, is their inability to supply a constant preload. A small change in the uniformity of the guide rails can significantly alter the amount of force developed in the bearing, changing the bearing flying height, which can cause instability and possibly derail the stage. These bearings are also less stiff, and the stage, therefore, is more prone to yaw. In addition, these mechanical preload devices generate a great deal of friction between the spring washers, conical bearing seat and ball bearing, which results in motion loss. Other associated problems include dynamic oscillations, such as pneumatic hammer instability, hysteresis and non-linearity.
One solution includes replacing the spring washers with an air cylinder, which would ensure a constant preload and eliminate the friction associated with the washers. This preload air bearing, however, still requires a ball bearing pivot, another source of friction, to compensate for any non-uniformity in the guide rails. In addition, such an air bearing may be difficult to implement due to packaging constraints. Thus, it would be advantageous to provide a preload hydrostatic bearing with a simple design that is capable of providing a constant force with minimum hysteresis to a fixed hydrostatic bearing despite variations in the guide rails.